<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
<style>
    .fade-enter,
    .fade-leave-to{opacity: 0; }
    .fade-enter-active,
    .fade-leave-active{transition: opacity .5s;}
</style>
<script src="./vue.js"></script>
</head>
<body>
    <div id="app">
        <transition name="fade" mode="out-in">
            <button :key="isoff "@click="isOff=!isOff">
                {{isOff ? 'Off' :'On'}}

            </button>
        
        </transition>

    </div>
    <script>
        var vm = new Vue({
            el:'#app',
            data:{ isOff:false}
        })
    </script>
</body>
</html>